bpf: verifier: Support KF_CHANGES_PKT flag

KF_CHANGES_PKT indicates that the kfunc call may change packet data.
This is analogous to bpf_helper_changes_pkt_data().

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>

Documentation/bpf/kfuncs.rst

@@ -200,6 +200,13 @@ single argument which must be a trusted argument or a MEM_RCU pointer.
 The argument may have reference count of 0 and the kfunc must take this
 into consideration.
 
+2.4.9 KF_CHANGES_PKT flag
+-----------------
+
+The KF_CHANGES_PKT is used for kfuncs that may change packet data.
+After calls to such kfuncs, existing packet pointers will be invalidated
+and must be revalidated before the prog can access packet data.
+
 2.5 Registering the kfuncs
 --------------------------
 

include/linux/btf.h

@@ -71,6 +71,7 @@
 #define KF_SLEEPABLE    (1 << 5) /* kfunc may sleep */
 #define KF_DESTRUCTIVE  (1 << 6) /* kfunc performs destructive actions */
 #define KF_RCU          (1 << 7) /* kfunc only takes rcu pointer arguments */
+#define KF_CHANGES_PKT  (1 << 8) /* kfunc may change packet data */
 
 /*
  * Return the name of the passed struct, if exists, or halt the build if for

kernel/bpf/verifier.c

@@ -8213,6 +8213,11 @@ static bool is_kfunc_rcu(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->kfunc_flags & KF_RCU;
 }
 
+static bool is_kfunc_changes_pkt(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->kfunc_flags & KF_CHANGES_PKT;
+}
+
 static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
 {
 	return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
@@ -9313,6 +9318,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			mark_btf_func_reg_size(env, regno, t->size);
 	}
 
+	if (is_kfunc_changes_pkt(&meta))
+		clear_all_pkt_pointers(env);
+
 	return 0;
 }